1. Field of the Invention
The present invention relates to a multi-beam type of scanning optical device used in an image forming apparatus such as a laser beam printer or a digital copying machine.
2. Description of the Related Art
In recent years, multi-beam-type scanning optical devices capable of simultaneously writing a plurality of lines by using a laser light source having a plurality of light emitting points have recently been developed for use in electrophotograhpic apparatuses, e.g., laser beam printers.
This type of scanning optical device enables scanning with a plurality of scanning laser beams simultaneously used, as described below. For example, as shown in FIG. 6A, two laser beams P1 and P2 as light beams are emitted from two light-emitting points 111 and 112 of a multi-beam laser unit, are each collimated into a parallel beam by a collimator lens 102, pass through a cylindrical lens 103 and an optical stop 104, irradiate to a reflecting surface 105a of a rotary polygon mirror 105, and travel through an fxcex8 lens system 106 to have an imaging point on a photosensitive member (photoconductor) 107 on a rotary drum.
Each of the two laser beams P1 and P2 incident upon the reflecting surface 105a of the rotary polygon mirror 105 is deflected by the mirror 105 to be scanned in a main scanning direction. Each beam moving in the main scanning direction by the rotation of the rotary polygon mirror 105 and moving in a sub-scanning direction by the rotation of the rotary drum forms an electrostatic latent image on the photosensitive member 107.
The cylindrical lens 103 condenses each of the laser beams P1 and P2 so that the beam is condensed into a linear shape on the reflecting surface 105a of the rotary polygon mirror 105. The cylindrical lens 103 and the fxcex8 lens system 106 form an optical face tangle error correction system to perform the function of preventing occurrence of an error in positioning of the above-mentioned imaging point in the sub-scanning direction on the photosensitive member 107 due to a face tangle error of the rotary polygon mirror 105. Also, the fxcex8 lens system 106 has the function of correcting the scanning movement of each beam so that the imaging point moves at a constant speed in the main scanning direction on the photosensitive member 107.
Writing with a plurality of beams P1 and P2 is thus performed to achieve high-speed, high-definition printing.
To reduce the spacing between the lines formed on the photosensitive member by the laser beams from the two light-emitting points of the laser unit, a method has been practiced in which the line connecting the two light-emitting points is set at an angle from a direction corresponding to the sub-scanning direction, that is, the two light-emitting points are shifted from each other in a direction corresponding to the main scanning direction, because there is a limit to the reduction between the distance between the two light-emitting points.
If the light-emitting points are positioned as described above, the necessary length of the reflecting surface 105a of the rotary polygon mirror 105 for simultaneously reflecting the plurality of beams P1 and P2 to perform scanning is increased, resulting in an increase in overall size of the rotary polygon mirror 105. As a solution of this problem, means for reducing the distance between the points at which the laser beams P1 and P2 are incident upon the rotary polygon mirror 105 has been devised. That is, the distance by which the laser beams P1 and P2 is reduced by reducing the distance between the rotary polygon mirror 105 and the optical stop 104 on the upstream side of the rotary polygon mirror 105. This arrangement is also effective in limiting a deterioration in image quality due to instability of focusing.
This arrangement will be described in more detail. The laser beam P1 emitted from the light-emitting point 111 is deflected by the rotary polygon mirror 105, passes through the fxcex8 lens system 106 and travels along a path L1 to have an imaging point at a position D on the photosensitive member 107. At this time, the laser beam P2 emitted from the light-emitting point 112 has an imaging point located just behind (or on the upstream side of) the position D in the main scanning direction indicated by an arrow B.
Thereafter, with rotation of the rotary polygon mirror 105 in the direction indicated by an arrow A (FIG. 6A shows the states of rotation of the rotary polygon mirror although the rotary polygon mirror is illustrated as if it is not rotated because the amount of rotation is extremely small), the laser beam P2 emitted from the light-emitting point 112 travels along a path L2 to reach the position D.
It is assumed here that the photosensitive member 107 moves to the position indicated by the broken line in FIG. 6A due to a reduction in the accuracy with which the photosensitive member 107 and the optical box incorporating the optical device are positioned. Since the laser beams P1 and P2 respectively emitted from the light-emitting points 111 and 112 reach the position D on the photosensitive member 107 with an angular difference of an angle xe2x80x9cxcex1xe2x80x9d from each other, the positions of the imaging spots on the photosensitive member 107 of the laser beams P1 and P2 shown by a broken line traveling along the paths L1 and L2 are spaced apart from each other by a distance xe2x80x9crxe2x80x9d. FIG. 6B is a diagram showing details of the encircled portion VI B of FIG. 6A.
If the position of the optical stop 104 is brought closer to the rotary polygon mirror 105 to reduce the angle xe2x80x9cxcex1xe2x80x9d, the distance xe2x80x9crxe2x80x9d between the imaging spots of the laser beams P1 and P2 on the photosensitive member 107, resulting from an error in positioning of the photosensitive member 107 as indicated by the broken line, is reduced. Thus, an increase in the length of the reflecting surface of the rotary polygon mirror and a deterioration in image quality resulting from an error in the imaging position due to instability of focusing can be suppressed.
However, if the number of scanning laser beams is increased, even the above-described technique is not a sufficiently effective solution of the problem of an increase in size of the reflecting surface for reflecting and scanning a plurality of laser beams, resulting in an increase in overall size of the polygon mirror and the problem of deterioration in image quality due to instability of focusing.
An object of the present invention is to provide a scanning optical device designed so as to prevent an increase in size of a deflecting scanning means due to an increase in length of a reflecting surface, and an image forming apparatus incorporating the scanning optical device.
Another object of the present invention is to provide a scanning optical device designed so as to prevent deterioration in image quality due to instability of focusing, and an image forming apparatus incorporating the scanning optical device.
Still another object of the present invention is to provide a scanning optical device and an image forming apparatus using the optical scanning device, the scanning device including a first light source unit for generating a plurality of light beams, a second light source unit for generating at least one light beam, and deflecting scanning means for deflecting by a reflecting surface the light beams generated by the first light source unit and the second light source unit to scan a member to be scanned, wherein the positions of the plurality of light beams generated by the first light source unit are different from each other in a direction corresponding to the direction of scanning performed by the deflecting scanning means on the reflecting surface of the deflecting scanning means, and the at least one light beam generated by the second light source unit is positioned between the plurality of light beams generated by the first light source unit in the direction corresponding to the scanning direction.